1. Field of the Invention
The present invention relates to a technique of suppressing adhesion of dust to the surface of an optical member (e.g., a solid-state image sensor, optical filter, or lens) which is mounted in an optical device (e.g., a digital camera) and arranged on or near the focal plane.
2. Description of the Related Art
Conventionally, if an alien substance such as dust adheres near the focal plane of the photographing lens of a lens interchangeable type single-lens reflex camera, the solid-state image sensor captures the shadow of the alien substance. The cause of the alien substance is considered to be dust entering from the outside in lens exchange, or fine powder generated from the wear of components (e.g., a resin serving as a structure material) generated along with the operation of the shutter or mirror inside the camera. The alien substance generated due to this cause sometimes enters between a cover glass for protecting especially the solid-state image sensor, and an optical filter (e.g., an infrared-cut filter or optical low-pass filter (to be abbreviated as an LPF hereinafter)) arranged in front of the cover glass. In this case, the camera must be disassembled to remove the alien substance. A closed structure is very effective in preventing an alien substance from entering between the cover glass of the solid-state image sensor and the optical filter.
If an alien substance adheres to a surface of the optical filter opposite to a surface facing the solid-state image sensor and the adhesion position is near the focal plane, the solid-state image sensor undesirably captures the shadow of the alien substance.
To solve this problem, there is known a structure of cleaning the cover glass surface of a solid-state image sensor with a wiper (Japanese Patent Laid-Open No. 2003-005254). This camera structure allows removing an alien substance adhered to the cover glass surface of the solid-state image sensor or the top surface (e.g., optical filter surface) of the dustproof structure without dismounting the lens or disassembling the camera. In the structure in Japanese Patent Laid-Open No. 2003-005254, the wiper wipes the cover glass surface of the solid-state image sensor or the top surface of the dustproof structure. When a solid alien substance such as metal powder adheres, it may scratch the cover glass surface of the solid-state image sensor or the top surface of the dustproof structure. The alien substance removed by the wiper floats in the camera, and the temporarily removed alien substance adheres again to the cover glass surface of the solid-state image sensor or on the top surface of the dustproof structure.
As a solution to this problem, there is known a technique of forming transparent electrodes on the cover glass surface of a solid-state image sensor and the surface of an optical filter in order to suppress adhesion of an alien substance on the cover glass surface of the solid-state image sensor (Japanese Patent Laid-Open No. 2000-029132). The technique disclosed in Japanese Patent Laid-Open No. 2000-029132 applies a potential to the transparent electrodes on the cover glass surface of the solid-state image sensor and the surface of the optical filter. The potential can neutralize static electricity generated on the cover glass surface of the solid-state image sensor and the surface of the optical filter, suppressing adhesion of dust. However, the transparent electrodes on the cover glass surface and optical filter surface decrease the light transmittance to the image sensor and generates an optically adverse effect. Static electricity generated on the cover glass surface of the solid-state image sensor and the surface of the optical filter varies depending on the ambient environment (temperature and humidity) and use conditions. It is difficult to neutralize static electricity. If neutralization of static electricity fails, adhesion of dust cannot be satisfactorily suppressed.
As another technique, there is known a technique of forming a periodic microstructure with a femtosecond laser on the surface of a flat silicon plate in order to decrease the adhesive force of minute particles in the air to the flat plate (Proceedings of the 2004 Autumn Annual Meeting of the Society of Powder Technology, Japan (FIGS. 1 and 2 on p. 34 and FIG. 4 on p. 35)). This reference reports that the periodic microstructure reduces the Van der Waals force and liquid bridge force acting between glass bead and the flat silicon plate. However, this reference does not describe whether the femtosecond laser can form a periodic microstructure on the surface of the cover glass of an optical element or that of an optical filter. Since the period of the periodic microstructure is about 620 nm within the visible light region, an optically adverse effect may occur. The technique of this report is not directly applicable as a measure against an alien substance adhered to the cover glass surface of a solid-state image sensor and the surface of an optical filter.
A microstructure is also formable on the substrate surface by arraying minute particles on the substrate surface. There is an arrangement in which the microstructure of minute particles functions as an antireflection film (Japanese Patent No. 3178009). The antireflection effect is achieved by forming a multilayered minute-particle thin film. Japanese Patent No. 3178009 discloses that a minute-particle film can provide an antireflection function and conduction function, but does not disclose any effect of reducing adhesive force.
The cover glass surface of a solid-state image sensor and the surface of an optical filter have dielectric multilayered films for preventing reflection and cutting infrared rays in order to obtain a desired spectral transmittance. However, forming a dielectric multilayered film on the top surface of the microstructure by vapor deposition or the like flattens the microstructure, losing the effect of suppressing adhesion of an alien substance.